1. Field of the Invention
The present invention relates to telecommunications cables. More particularly, the present invention relates to telecommunications cables having optical fibers buffered by thermoplastic polyolefin elastomers.
2. Discussion of Related Art
In telecommunications cables having transmission elements such as optical fibers coated with an ultraviolet light curable coating, the transmission elements are typically protected, either individually or as a group, by a buffering material. The relationship between the buffering material and the transmission elements is usually described as either "tight," "near-tight" or "loose." For example, a "tight" buffered optical fiber has buffer material extruded directly over the coated optical fiber. Optical fibers which are "near-tight" buffered have a hollow buffer member, often a tube with an inner diameter slightly larger (up to about 0.4 mm) than the outer diameter of optical fiber or the approximate outer diameter of a plurality fibers contained therein. For example, "near-tight" buffered optical fiber units typically consist of one or a plurality of optical fibers surrounded by a sheath or buffer unit of a flexible material. The flexible buffer unit has a wall thickness of approximately 0.2 mm and an inner radius less than 0.3 mm greater than that of the fiber bundle contained within. More particularly, for example a 4 fiber flexible buffer tube has an outside diameter (OD) of 1.0 mm and an inside diameter (ID) of 0.6 mm. A 6 fiber flexible buffer unit has an OD of 1.15 mm and an ID of 0.75 mm. A 12 fiber flexible buffer unit has an OD of 1.4 mm and an ID of 1.0 mm. Use of such compact buffer units allows the design of cables with overall smaller O.D. and lower weight per unit length. These units also allow a high fiber density in smaller diameter cables.
In a "loosely" buffered optical fiber cable, the inside diameter of the buffer tube is substantially larger (greater than about 0.4 mm) than the outside diameter of the optical fiber or the approximate outside diamter of a bundle of optical fibers. Loosely buffered optical fiber cable designs are well known in the art.
For tight and near-tight buffering applications, a low modulus of elasticity may be desired in order to ensure easy access and handling of buffer units without damaging fibers encapsulated therein. Tight and near-tight buffer members have more contact with the coated fibers. In order to achieve better distribution of contact forces on the coated fiber, a specific range of modulus is required.
Buffer materials typically used for making buffer tubes for buffering optical fibers in a telecommunications cable include plasticized polyvinyl chloride (PVC), polybutylene terephtalate (PBT), polycarbonate (PC) and some polyolefins, for example. The aforementioned materials have advantages and disadvantages when used for buffering optical fibers. For example, some of these materials, such as PC and PBT, are more expensive. Some, such as PVC, do not provide an acceptable modulus of elasticity within the range of temperatures a telecommunications cable may be exposed to during operation. To overcome some of the problems of PVC, some designs use plasticized PVC. However, plasticized PVC may still show a considerable increase in modulus at low temperature. Also, another drawback of plasticized PVC is that plasticizers in the PVC can be leached out by many thixotropic, water blocking gels that are disposed in the buffer tube with the fiber or fibers. Leaching causes an increase in buffer unit modulus and rigidity as well as possible degradation of encapsulated fiber properties. This leaching is most prevalent in polyolefin and polyol based gels. To minimize plasticizer leaching, special grades of plasticized PVC with migration resistant plasticizers and compatible water blocking gels, such as silicone based gels, must be used. Silicone gels, however, much more costly than polyol or polyolefin gels.
The solutions to the leaching problem are relatively expensive and do not totally eliminate the problem. Even if such solutions are used, migration or extraction of plasticizers can still occur through interaction between the fiber coating and buffer member.